The invention relates to electronic image sensors and more particularly, those which work on the basis of active pixels in MOS technology. The invention relates notably to a method for controlling the various transistors that make up the pixels. The active pixels usually comprise a photodiode and three, four or five MOS transistors making it possible to control the reading of the charges generated by the light in the photodiode. The pixels with three transistors work by directly transferring to a column conductor the potential of the photodiode, a potential which varies according to the lighting and the light integration time. The pixels with four transistors work by first transferring from the photodiode to a capacitive conversion node the charges generated by the light, then by referring the potential of the conversion node to a column conductor; one of the transistors is used to reset the potential of the conversion node before the charge transfer from the photodiode to the conversion node. The pixels with five transistors also include a transistor for resetting the potential of the photodiode. This invention relates to a classical 3T pixel concept with transistors added to implement further functionality.
It is desired that the sensor should capture images that have the widest possible dynamic range, that is to say, pixels that generates a detectable signal in the very low lighting condition but also are capable of receiving very luminous imaging condition without saturation. A number of solutions have been sought to obtain a wide dynamic range, that is reducing the noise floor for low light condition and increasing the maximum charge capacity for highly illuminated images.
One solution consists in using a successive capture of a number of images with different integration times. If the signal supplied by a pixel that has undergone a long integration time is saturated, it is replaced by a signal from the same pixel, having undergone a short integration time. This presupposes taking several successive images and the overall acquisition time is long. Furthermore, the images have to be processed pixel by pixel in order to choose the most suitable signal for each before going on to a next image.
Another solution consists in having a mixed matrix with small pixels and large pixels. The small pixels, less sensitive, are used if there is a lot of light. A complex suitable processing is required and the overall resolution of the matrix is reduced.
Yet another solution consists in measuring the time that it takes a pixel to arrive at saturation to deduce therefrom information concerning the level of light in the presence of saturating lighting. This presupposes more complex pixels.
Solutions with pixels with logarithmic or linear-logarithmic function or with response curve slope variation have also been proposed for pixels with three transistors. These rely on a variation of the potential of the gate of the transistor for resetting the photodiode. These solutions are sensitive to technological dispersions: dispersion of threshold voltages of the transistors of the various pixels and dispersion of the no-load potential of the photodiode after reset.
WO99/34592 proposes a device whose read circuit comprises a first capacitor for storing a potential level for resetting the storage node of the pixel, a second capacitor for storing a potential level taken by the storage node after a first integration period, a third capacitor for storing a potential level taken by the storage node after a second integration period following the first but much shorter than the first, and a threshold circuit for comparing the potential level in the first capacitor with a threshold and using the potential stored in the second capacitor rather than in the first in the case where, as a result of excessively strong lighting, the threshold would be exceeded. This device requires three sampling in-pixel capacitors. Now, the sampling capacitors occupy a very large surface area in the read circuit (around 15% of the surface area for each capacitor). Also, the matrix image sensors are highly sensitive to an effect which is the fixed read noise in column mode. This noise results from the offset dispersions of the column amplifiers and is reflected in parallel vertical lines which are very visible to the eye when the images are displayed. There are methods for reducing it, but these methods do not apply if there are three capacitors. It should also be noted that if this noise is not eliminated, it is ultimately multiplied by the ratio of the integration periods in the case where the second capacitor is used rather than the first.
In the prior art also modified three-transistor (3T) CMOS image sensors pixels have been reported which have two operational modes, i.e. one for low-intensity images and one for high-intensity images. These modified 3T-CMOS image sensors are provided with an extra transistor (thus turning it into a 4T-CMOS image sensor) to implemented a dual full-well characteristic. The pixel charge capacity can be increased from low-full well to high full-well by increasing the capacitance of the pixel from a low value to a high value. The photo-diode in such circuit is preferably designed in such way that it only collects charge and does not store these charges. The storage is performed in the parallel added linear capacitors. This is achieved by using a largely-pinned photo-diode with a pinning potential with is preferably below a lower limit (for example 1V) of the linear range (working regime) of the in-pixel potential. The upper limit of this linear range is defined by the reset voltage (for example 3V). The selection of the respective mode, low full-well or high full-well is static. Such selection is typically made depending on the application. The advantage of the low full-well mode is the low noise floor generated within the pixel. The disadvantage of the low full-well mode is the low maximum pixel charge. The advantage of the high full-well mode is the large maximum pixel charge, the disadvantage of the high full-well mode is the higher noise floor.
The above-mentioned solutions all have their advantages and disadvantages. There is still a need for a non-complex solution which combines advantages of the various solutions mentioned without compromising too much on other features.